Anions of sulfonyl imide type, by virtue of their very low basicity, are increasingly prevalent in the field of energy storage in the form of inorganic salts in batteries or organic salts in supercondensers or in the field of ionic liquids. Since the battery market is in full expansion and the reduction of battery manufacturing costs is becoming a major challenge, a process for large-scale and low-cost synthesis of anions of this type is necessary.
In the specific field of Li-ion batteries, the salt currently most commonly used is LiPF6, but this salt shows numerous disadvantages, such as limited thermal stability, instability to hydrolysis and therefore lower safety of the battery. Recently, new salts having the FSO2− group have been studied and have demonstrated numerous advantages, for instance better ionic conductivity and resistance to hydrolysis. One of these salts, LiFSI (LiN(FSO2)2), has shown very advantageous properties which make it a good candidate for replacing LiPF6.
Few processes for the synthesis of LiFSl or of its corresponding acid have been described, but it is clearly apparent that, in all these processes, the key step is the S—F bond formation step.
The first synthesis route described (Appel & Eisenbauer, Chem Ber. 95, 246-8, 1962) consists in reacting fluorosulfonic acid (FSO3H) with urea. The use of FSO3H thus makes it possible to already have the S—F bond formed, but the corrosive and toxic nature of this product did not allow industrialization of the process.
A second route (Ruff & Lustig, Inorg. Synth. 1968, 11, 138_43) consists in, firstly, synthesizing a dichlorinated compound having the following formula (ClSO2)2NH, and then performing chlorine/fluorine exchange with AsF3. However, this process is not industrializable owing to the high price and the toxicity of AsF3.
Document WO 02/053494 describes a third route which consists of a Cl/F exchange on (ClSO2)2NH using a fluoride of a monovalent cation which may be an alkali metal cation or a cation of onium type (NR4+), in an aprotic solvent. According to said document, the reaction proves to be very slow.
Example 10 of document WO 07/068822 describes the synthesis of bis(fluorosulfonyl)imide in anhydrous hydrofluoric acid. Thus, the reaction is carried out in an autoclave with 1 g of bis(chlorosulfonyl)imide and 4 g of anhydrous HF at various reaction temperatures and times. Said document teaches that, even at temperatures of 130° C., the reaction yield does not exceed 55%. In addition, it teaches that the presence of impurities makes the separation difficult on an industrial scale. Said document concludes that the synthesis of bis(fluorosulfonyl)imide using HF is not satisfactory, and recommends the use of a lithium fluoride.
Despite this preconception, the applicant has developed a process for manufacturing fluorinated compounds comprising at least one fluorosulfonyl group (including bis(fluorosulfonyl)imide) by making use of anhydrous hydrofluoric acid. This process has the advantage of being easy to extrapolate to an industrial scale and HF also has the advantage of being inexpensive.